It has been common practice to use amorphous silicon or polysilicon as the semiconducting material for thin-film transistors.
These inorganic semiconducting materials typified by silicon materials need high-temperature treatment when they are made into thin film. Therefore, they cannot be applied to plastic substrates. Moreover, they need vacuum processing, which requires expensive equipment and hence leads to high production cost.
A new material has recently appeared which is expected to overcome the above-mentioned disadvantage of inorganic semiconducting materials. It is the organic semiconducting material, which is attracting general attention.
Replacing inorganic semiconducting materials with organic ones realizes large area devices easily and economically; it also makes it possible to form devices on a plastic film, thereby giving rise to flexible devices. Flexible devices will realize electronic paper. For example, newspaper in the form of flexible display will greatly reduce the consumption of paper and hence reduce the load on environment.
Applications of organic semiconducting materials which are currently under active investigation include thin-film transistors based on tetrathiafulvalene derivatives (see JP-A 2004-288836, JP-A 2006-245131, JP-A 2005-223175, JP-A 2007-42717, and JP-A 2008-94781, and Chem. Mater., 19, 6382, 2007 (hereinafter referred to as Non-Patent Document 1)).
Noticeable among these derivatives is hexamethylenetetrathiafulvalene (HMTTF for short hereinafter), which reportedly exhibits as high mobility as 11.2 cm2/Vs measured in the form of single-crystal transistor (see Non-Patent Document 1).
Unfortunately, this HMTTF possesses the strong donor property and three-dimensional mutual action and hence it merely gives rise to a thin film with unstable film quality. The transistor made from it is poor in characteristic properties.